Optical element and optical unit using the same

ABSTRACT

An optical element of the present invention is provide with a first surface which is formed on the front-object side and upon which light from the front-object side is incident, a second surface formed on the image side, and a third surface which is formed between the first surface and the second surface and upon which light from the generally lateral object side is incident. The first surface is provided with a first transmission surface formed around the optical axis and a first reflection surface which faces the image side and which is formed annularly around the first transmission surface. The second surface is provided with a second transmission surface formed around the optical axis, and a second reflection surface which faces the front object side and which is formed annularly around the second transmission surface.

TECHNICAL FIELD

This invention relates to an optical element by which a front object andan approximately lateral object can be simultaneously observed and to anoptical unit using the optical element.

BACKGROUND ART

Optical units which are provided with an optical system by which a frontobject and an approximately lateral object can be simultaneouslyobserved and a lens barrel that holds the optical system have been knownup to now. In this case, the range which the above term, “approximatelylateral”, defines includes not only the lateral side of the opticalsystem itself but also the diagonally forward and diagonally backwardsides of the optical system.

In optical systems for such optical units, an optical system which isformed in such a way that, after light from theapproximately-lateral-object side is reflected two times on the inside,the light emerges to the image side to be guided to an imaging elementlike CCD (Charge Coupled Device), CMOS (Complementary Metal-OxideSemiconductor), or the like has been known (for example, refer toInternational Publication No. 2003/042743).

DISCLOSURE OF INVENTION

However, in the optical system which is described in InternationalPublication No. 2003/042743, after light from theapproximately-lateral-object side is reflected by two members, the lightemerges to the image side. Accordingly, the optical system which isdescribed in International Publication No. 2003/042743 has a problemthat, in the case where cumulative tolerance between these members andthe lens barrel that holds these members becomes large, one of thesemembers easily becomes eccentric to the other of these members inassembling the optical unit and the capability of forming an imageeasily deteriorates.

The present invention is made in view of such a conventional technicalproblem. The object of the present invention is to offer an opticalelement and an optical unit which can restrain the deterioration of thecapability of forming an image.

In order to achieve the above-described object, an optical element ofthe present invention is characterized in that the optical element isprovided with a first surface which is formed on the front-object sideand through which light from the front-object side enters, a secondsurface which is formed on the image side, and a third surface which isformed between the first and second surfaces and through which lightfrom the approximately-lateral-object side enters, wherein the firstsurface is provided with a first transmission surface which is formedwith the center of the first transmission surface being on the opticalaxis and a first reflection surface which faces toward the image sideand is formed around the first transmission surface and in the shape ofa ring, and the second surface is provided with a second transmissionsurface which is formed with the center of the second transmissionsurface being on the optical axis and a second reflection surface whichfaces toward the front-object side and is formed around the secondtransmission surface and in the shape of a ring.

Also, in an optical element of the present invention, it is preferredthat: after light from the front-object side is incident on the firsttransmission surface, the light emerges from the second transmissionsurface to the image side; and after light from theapproximately-lateral-object side is incident on the third surface, thelight is reflected by the second reflection surface and the firstreflection surface in that order and emerges from the secondtransmission surface to the image side.

Also, in order to achieve the above-described object, an optical unit ofthe present invention is characterized in that the optical unit isprovide with an optical system by which a front object and anapproximately lateral object are observed, and a lens barrel which holdsthe optical system, wherein the optical system includes one of theabove-described optical elements, the lens barrel at least has a firstlens barrel and a second lens barrel, the first lens barrel is fitted tothe first surface of the optical element, and the second lens barrel isfitted to the second surface of the optical element.

Also, in an optical unit of the present invention, it is preferred thatthe external peripheral surface of the optical unit is composed of theexternal peripheral surface of the lens barrel and the third surface.

Also, in an optical unit of the present invention, it is preferred thatthe optical system comprises a first optical system and a second opticalsystem, the first optical system is provided with a first lens grouphaving negative refractive power, a second lens group including theoptical element, and a third lens group in order from the front-objectside, and the second optical system is provided with the second lensgroup and the third lens group in order from theapproximately-lateral-object side.

Also, in an optical unit of the present invention, it is preferred thata first fitting portion is formed around the first reflection surface ofthe optical element, a second fitting portion is formed around thesecond reflection surface of the optical element, the first lens barrelis fitted to the first fitting portion, and the second lens barrel isfitted to the second fitting portion.

Also, in an optical unit of the present invention, it is preferred thatat least one of the first and second fitting portions is formed in theshape of a step.

Also, in an optical unit of the present invention, it is preferred thatthe first and second fitting portions are formed in such a way that thefirst and second fitting portions are rotationally symmetric relative tothe optical axis.

Also, in an optical unit of the present invention, it is preferred thatthe first lens barrel includes a first holding portion, the second lensbarrel includes a second holding portion, the first holding portionholds the optical element through the first surface of the opticalelement, the second holding portion holds the optical element throughthe second surface of the optical element, and at least one of the firstand second holding portions is formed in the shape of a step.

Also, in an optical unit of the present invention, it is preferred thatthe first and second holding portions are formed in such a way that thefirst and second holding portions are rotationally symmetric relative tothe optical axis.

Also, in order to achieve the above-described object, an optical unit ofthe present invention is characterized in that the optical unitcomprises an optical system by which a front object and an approximatelylateral object are observed, a lens barrel which holds the opticalsystem, and an imaging element, wherein the optical system includes alens group which includes the optical element and a lens group which isarranged nearer to the image side than the lens group including theoptical element and includes an aperture stop, and the optical unit isprovided with an annular light-shielding member which is placed in thevicinity of the aperture stop, is made of light-shielding material, andhas an internal diameter that is larger than the internal diameter ofthe aperture stop and smaller than the internal diameter of the lensbarrel.

Also, in an optical unit of the present invention, it is preferred thatthe aperture stop is formed integratedly with the light-shieldingmember.

Also, in an optical unit of the present invention, it is preferred thatthe imaging element is held by the lens barrel.

Also, in an optical unit of the present invention, it is preferred thatthe lens barrel consists of: an internal barrel which holds a lens groupthat is arranged nearer to the image side than the optical element; andan external barrel which holds the internal barrel, and thelight-shielding member is formed integratedly with the internal barrel.

Also, in an optical unit of the present invention, it is preferred thatthe external diameter of the light-shielding member approximatelycorresponds with the internal diameter of the lens barrel.

According to the present invention, it is possible to offer an opticalelement and an optical unit which can restrain the deterioration of thecapability of forming an image.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view showing the optical unit according to anembodiment 1 of the present invention.

FIG. 2 is a schematic view showing an optical element which included byan optical system that is provided for the optical unit that is shown inFIG. 1.

FIG. 3 is a schematic view showing a variation of the optical unitaccording to the embodiment 1 of the present invention.

FIG. 4 is a schematic view showing a variation of the optical unitaccording to the embodiment 1 of the present invention.

FIG. 5 is a schematic view showing a variation of the optical unitaccording to the embodiment 1 of the present invention.

FIG. 6 is a schematic view showing a variation of the optical unitaccording to the embodiment 1 of the present invention.

FIG. 7 is a schematic view showing the optical unit according to anembodiment 2 of the present invention.

FIG. 8 is a schematic view showing a variation of the optical unitaccording to the embodiment 2 of the present invention.

FIG. 9 is a schematic view showing a variation of the optical unitaccording to the embodiment 2 of the present invention.

FIG. 10 is a schematic view showing a variation of the optical unitaccording to the embodiment 2 of the present invention.

FIG. 11 is a schematic view showing a variation of the optical unitaccording to the embodiment 2 of the present invention.

FIG. 12 is a schematic view showing an endoscope apparatus into which anoptical unit of the present invention is incorporated.

BEST CONFIGURATION FOR EMBODYING INVENTION

The embodiments of the present invention will be explained in detailbelow.

Embodiment 1

First, the constitution of the optical unit of the present embodiment isexplained using FIG. 1. The optical unit of the present embodimentcomprises: an optical system which consists of three lens groups and bywhich a front object and an approximately lateral object can besimultaneously observed; and two lens barrels which hold the opticalsystem. Besides, an imaging element like CCD, CMOS, or the like isarranged at the rear of the optical system (on the image side), althoughthe imaging element is not shown in the drawings in the presentembodiment. Also, low pass filter which is given IR cut coating,CCD-cover glass, or the like may be arranged between the optical systemand the imaging element.

The optical system of the present embodiment by which a front object andan approximately lateral object can be simultaneously observed includes,in order from the front-object side, a first lens group G₁ through whichlight from the front-object side enters, a second lens group G₂ throughwhich light transmitted by the lens group G₁ and light from theapproximately-lateral-object side enter, and a third lens group G₃. Inthis case, the first lens group G₁ has negative refractive power. Also,the third lens group G₃ has positive refractive power.

The first lens group G₁ is composed of a lens L₁ which is aplano-concave lens the concave surface of which faces toward the imageside. The second lens group G₂ is composed of: a lens L₂₁ which is anoptical element for simultaneously observing a front object and anapproximately lateral object and has a special shape; and a plano lensL₂₂ to which an aperture stop is integratedly fitted on the image side,in that order from the front-object side. The third lens group G₃ iscomposed of: a plano-convex lens L₃₁ the convex surface of which facestoward the image side; a cemented lens which consists of a biconvex lensL₃₂ and a biconcave lens L₃₃; a biconvex lens L₃₄ the image-side surfaceof which is an aspherical surface; and a plano lens L₃₅, in that orderfrom the front-object side. Besides, the shapes of the lenses except thelens L₂₁ are not limited to the above-described shapes.

Now, the lens L₂₁ which is an optical element for simultaneouslyobserving a front object and an approximately lateral object and has aspecial shape is explained in detail using FIG. 2. The lens L₂₁ has: afirst surface L₂₁a which is formed on the front-object side and throughwhich light from the front-object side enters; a second surface L₂₁bwhich is formed on the image side; and a third surface L₂₁c which isformed in the circumferential direction so as to surround the opticalaxis between the first surface L₂₁a and the second surface L₂₁b andthrough which light from the approximately-lateral-object side enters.This third surface L₂₁c may be formed on a part of or on the whole ofthe surface in the circumferential direction between the first surfaceL₂₁a and the second surface L₂₁b. Also, a first fitting portion L₂₁d isformed between the first surface L₂₁a and the third surface L₂₁c and asecond fitting portion L₂₁e is formed between the second surface L₂₁band the third surface L₂₁c.

The first surface L₂₁a of the lens L₂₁ includes: a first transmissionsurface L₂₁a₁ which is formed with the center of the first transmissionsurface L₂₁a₁ being on the optical axis; and a first reflection surfaceL₂₁a₂ which faces toward the image side and is formed around the firsttransmission surface L₂₁a₁ and in the shape of a ring. The secondsurface L₂₁b includes: a second transmission surface L₂₁b₁ which isformed with the center of the second transmission surface L₂₁b₁ being onthe optical axis; and a second reflection surface L₂₁b₂ which facestoward the front-object side and is formed around the secondtransmission surface L₂₁b₁ and in the shape of a ring. Besides, in thepresent embodiment, the third surface L₂₁c is formed in such a way thatthe image-side diameter of the third surface L₂₁c is larger than thefront-object-side diameter of the third surface L₂₁c. However, the thirdsurface L₂₁c may be formed in such a way that the image-side diameter ofthe third surface L₂₁c is smaller than the front-object-side diameter ofthe third surface L₂₁c. In this case, the above word, “thefront-object-side diameter”, means “the diameter in the surface which islocated at the position nearest to the front-object side in the thirdsurface L₂₁c and is perpendicular to the optical axis”. Also, the aboveword, “the image-side diameter”, means “the diameter in the surfacewhich is located at the position nearest to the image side in the thirdsurface L₂₁c and is perpendicular to the optical axis”.

Besides, the first reflection surface L₂₁a₂ and the second reflectionsurface L₂₁b₂ are formed through vapor deposition. Specifically, forexample, after a mask which has the same shape as that of the firsttransmission surface L₂₁a₁ is put on the first transmission surfaceL₂₁a₁, the whole of the first surface L₂₁a is given mirror coating, andthen, the mask is peeled off from the first transmission surface L₂₁a₁.A masked part of the first surface L₂₁a is not given mirror coating bythe use of such a method, so that the first transmission surface L₂₁a₁can be used as a transmission surface even after the first reflectionsurface L₂₁a₂ is formed. Besides, methods for forming the firstreflection surface L₂₁a₂ and the second reflection surface L₂₁b₂ are notlimited to the above-described manner.

Also, a first fitting portion L₂₁d is formed around the first reflectionsurface L₂₁a₂ of the first surface L₂₁a of the lens L₂₁. The firstfitting portion L₂₁d is fitted to a first lens barrel T₁, as shown inFIG. 1. Besides, the first fitting portion L₂₁d is formed by acylindrical surface L₂₁d₁ the center of which is on the optical axis andan annular surface L₂₁d₂ which is approximately perpendicular to theoptical axis, in such a way that the first fitting portion L₂₁d isformed annularly and in the shape of a step. On the other hand, a secondfitting portion L₂₁e to which a second lens barrel T₂ holding the secondlens group G₂ is fitted as shown in FIG. 1 is formed around the secondreflection surface L₂₁b₂ of the second surface L₂₁d of the lens L₂₁. Asdescribed above, the fitting portion for fitting the lens barrel isprovided for the lens L₂₁ which is an optical system for simultaneouslyobserving a front object and an approximately lateral object in theoptical unit of the present embodiment, so that there is no necessityfor providing an optical unit with a holding member that is larger thanthe diameters of lenses of its optical system as in conventional opticalunits. In this case, the first lens barrel T₁, the lens L₂₁, and thesecond lens barrel T₂ are arranged in such a way that each of the firstlens barrel T₁, the lens L₂₁, and the second lens barrel T₂ isrotationally symmetric relative to the optical axis. In addition, thefirst fitting portion L₂₁d is also formed in such a way that the firstfitting portion L₂₁d is rotationally symmetric relative to the opticalaxis.

Next, paths which light rays having entered the optical system of theoptical unit of the present embodiment follow are explained using FIGS.1 and 2. Light rays LC₁ which enters the optical system of the opticalunit of the present embodiment from the front-object side first passthrough a lens L₁. And, the light rays LC₁ which have passed through thelens L₁ enter the first transmission surface L₂₁a₁ of the first surfaceL₂₁a of the lens L₂₁. Thereafter, the light rays LC₁ which have enteredthe first transmission surface L₂₁a₁ emerge from the second transmissionsurface L₂₁b₁ of the second surface L₂₁b of the lens L₂₁. The light raysLC₁ which have emerged from the second transmission surface L₂₁b₁ passthrough a lens L₂₂, a lens L₃₁, a lens L₃₂, a lens L₃₃, a lens L₃₄, anda lens L₃₅, in that order, and then the light rays LC₁ enter an imagingelement. As a result, the image of the front object is formed on theimaging element.

On the other hand, light rays LC₂ which enters the optical system of theoptical unit of the present embodiment from theapproximately-lateral-object side first enters the third surface L₂₁c ofthe lens L₂₁. And, the light rays LC₂ which have entered the thirdsurface L₂₁c are reflected to the object side by the second reflectionsurface L₂₁b₂ of the second surface L₂₁b of the lens L₂₁. Thereafter,the light rays LC₂ which have reflected by the second reflection surfaceL₂₁b₂ are reflected to the image side by the first reflection surfaceL₂₁a₂ of the first surface L₂₁a of the lens L₂₁. Further thereafter, thelight rays LC₂ which have reflected by the first reflection surfaceL₂₁a₂ emerge from the second transmission surface L₂₁b₁ of the secondsurface L₂₁b of the lens L₂₁. The light rays LC₂ which have emerged fromthe second transmission surface L₂₁b₁ pass through the lens L₂₂, thelens L₃₁, the lens L₃₂, the lens L₃₃, the lens L₃₄, and the lens L₃₅, inthat order, and then the light rays LC₂ enter the imaging element. As aresult, the image of the approximately lateral object is formed on theimaging element.

The lens group G₂ of the optical system that constitutes the opticalunit of the present embodiment includes the lens L₂₁. And, this lens L₂₁is an optical element having the first reflection surface L₂₁a₂ and thesecond reflection surface L₂₁b₂. Accordingly, as a result of making thelens L₂₁ with sufficient precision, the relation between the tworeflection surfaces arranged also has sufficient precision, so that itis possible to prevent the first reflection surface L₂₁a₂ from becomingeccentric to the second reflection surface L₂₁b₂ in assembling theoptical unit. As a result, in optical units for which optical elementslike the lens L₂₁ in the present embodiment are used, it is hard for onemember (the first reflection surface L₂₁a₂) to become eccentric to theother member (the second reflection surface L₂₁b₂) as in the prior art,and it is possible to prevent the deterioration of a capability offorming an image.

Also, every one of the first lens barrel T₁, the lens L₂₁, and thesecond lens barrel T₂ is arranged in such a way that the every one isrotationally symmetric to the optical axis. As a result, the opticalaxes of the first lens group G₁ held by the first lens barrel T₁, thesecond lens group G₂ including the lens L₂₁, and the second lens groupG₂ held by the second lens barrel T₂ can be easily made to correspondwith one another in assembling the optical unit. Accordingly, it ispossible to prevent a plurality of the lenses from becoming eccentric toone another, and it is possible to prevent the deterioration of thecapability of forming an image.

Also, the first fitting portion L₂₁d is also formed in such a way thatthe first fitting portion L₂₁d is rotationally symmetric to the opticalaxis. Accordingly, an adjustment of eccentricity is easily made whilethe first lens barrel T₁ and the lens L₂₁ are being rotated about theoptical axis in assembling the optical unit. Accordingly, in the opticalunit of the present embodiment, it is hard for the first lens barrel T₁and the lens L₂₁ to become eccentric to each other, and it is possibleto prevent the deterioration of the capability of forming an image dueto eccentricity.

Besides, in the above-described embodiment, two lens barrels are fittedto fitting portions which are formed in an optical element,respectively. However, the two lens barrels T₁ and T₂ may be fitteddirectly to the non-reflection surfaces on the back sides of the tworeflection surfaces of the lens L₂₁ that is the optical element,respectively, without forming the fitting portions in the opticalelement.

Also, in the above-described embodiment, the first fitting portion L₂₁dis formed in the shape of a step. However, there is not necessity tonecessarily form the first fitting portion L₂₁d in such a shape. Forexample, like a first fitting portion L₂₁d′ in a variation of theoptical unit according to the present invention which is shown in FIG.3, a first fitting portion may be formed in such a way that theobject-side surface of the first fitting portion has no difference inlevel from the first surface L₂₁a.

Also, in the above-described embodiment, the step-shaped first fittingportion L₂₁d is formed on the front-object side of the optical element.However, there is not necessity to necessarily form such a fittingportion on the front-object side of the optical element. As in avariation of the optical unit according to the present invention whichis shown in FIG. 4, an optical element may be composed of the lens L₂₁and the lens L₂₂ which are included by the second lens group G₂, so thata step-shaped second fitting portion L₂₁₋₂₂e is formed on the image sideof the optical element.

Besides, in this variation, the second fitting portion L₂₁₋₂₂e is formedin such a way that the second fitting portion L₂₁₋₂₂e is rotationallysymmetric to the optical axis. As a result, the second lens barrel T₂and the optical element can be easily touched to each other to beassembled. Accordingly, in the optical unit of the present variation, itis hard for both of them to become eccentric, and it is possible toprevent the deterioration of the capability of forming an image due toeccentricity.

Also, in this variation, the step-shaped second fitting portion L₂₁₋₂₂eis formed on the image side of the optical element by making the opticalelement of two lenses. However, by the use of the same method as theabove-described method of forming the first fitting portion L₂₁d in theembodiment, a step-shaped second fitting portion may be formed in onelens, for example, only in the lens L₂₁.

Also, in the above-described embodiment and variation, the step-shapedfitting portion is formed only on either of the object side or the imageside of the optical element. However, like the first fitting portionL₂₁d and the second fitting portion L₂₁₋₂₂e in a variation of theoptical unit according to the present invention which is shown in FIG.5, fitting portions may be formed in the both surfaces of the opticalelement.

Besides, in this variation, every one of the first fitting portion L₂₁dand the second fitting portion L₂₁₋₂₂e is formed in such a way that theevery one is rotationally symmetric to the optical axis. As a result,the first lens barrel T₁ and the second lens barrel T₂ can be easilytouched to the first fitting portion L₂₁d and the second fitting portionL₂₁₋₂₂e of the optical element respectively to be assembled.Accordingly, in the optical unit of the present variation, it is hardfor the optical element, the first lens barrel T₁, and the second lensbarrel T₂ to become eccentric, and it is possible to prevent thedeterioration of the capability of forming an image due to eccentricity.

Also, in the above-described embodiment and variation, the fittingportion is formed in the optical element. However, instead of formingthe fitting portion in the optical element, a holding portion may beformed in one of or both of a surface of the first lens barrel which isfitted to the first surface of the optical element and a surface of thesecond lens barrel which is fitted to the second surface of the opticalelement. For example, as in a variation of the optical unit according tothe present invention which is shown in FIG. 6, a first holding portionT₁a may be formed on the edge of the first lens barrel T₁ on thelens-L₂₁ side, and a second holding portion T₂a may be formed on theedge of the second lens barrel T₂ on the front-object side.

Besides, in this variation, the first holding portion T₁a and the secondholding portion T₂a are formed in the shape of a step and in such a waythat the first holding portion T₁a and the second holding portion T₂aare rotationally symmetric to the optical axis. As a result, the firstholding portion T₁a and the second holding portion T₂a can be easilytouched to the optical element to be assembled. Accordingly, in theoptical unit of the present variation, it is hard for the opticalelement, the first lens barrel T₁, and the second lens barrel T₂ tobecome eccentric, and it is possible to prevent the deterioration of thecapability of forming an image due to eccentricity.

Embodiment 2

Next, the optical unit according to the embodiment 2 will be explainedin detail using FIG. 7. Besides, because the optical system whichconstitutes the optical unit of the present embodiment has approximatelythe same constitution as that of the optical system of theabove-described embodiment 1, the explanations of the lens L₂₁ that isthe optical element and the paths which light rays having entered thisoptical system follow are omitted.

This optical unit comprises an optical system which consists of threelens groups, an annular light-shielding member which is inserted intothe optical system, an imaging element having an image plane on which animage is formed by the optical system, and a lens barrel which holds theoptical system, the light-shielding member, and the imaging element.Besides, an imaging element like CCD, CMOS, or the like is arranged atthe rear of the optical system (on the image side), although the imagingelement is not shown in the drawings in the present embodiment. Also,low pass filter which is given IR cut coating, CCD-cover glass, or thelike may be arranged between the optical system and the imaging element.

The optical system in the present embodiment by which a front object andan approximately lateral object can be simultaneously observed includes,in order from the front-object side, a first lens group G₁ which hasnegative refractive power, a second lens group G₂, and a third lensgroup G₃ which has positive refractive power. Besides, the third lensgroup G₃ is provided with an aperture stop S.

And, in light entering the optical system, light from the front-objectside enters the first lens group G₁, the second lens group G₂, and thethird lens group G₃, in that order. On the other hand, light from theapproximately-lateral-object side enters the second lens group G₂ andthe third lens group G₃, in that order.

Also, this optical system is formed in such a way that the opticalsystem is a retro-focus type optical system for both light from thefront-object side and light from the approximately-lateral-object side.The use of such a constitution of the optical system can make an angleof view for observation large.

The first lens group G₁ is composed of a lens L₁₁ which is aplano-concave lens the concave surface of which faces toward the imageside. The second lens group G₂ is composed of a cemented lens whichconsists of: a lens L₂₁ that is an optical element for simultaneouslyobserving a front object and an approximately lateral object and has aspecial shape; and a lens L₂₂ that is a negative meniscus lens theconvex surface of which faces toward the front-object side, in thatorder from the front-object side. The third lens group G₃ is composedof: a lens L₃₁ which is a plano lens; a lens L₃₂ which is a plano-convexlens the convex surface of which faces toward the image side; a lens L₃₃which is a biconvex lens; a lens L₃₄ which is a biconcave lens; a lensL₃₅ which is a biconvex lens the image-side surface of which is anaspherical surface; and a lens L₃₆ which is a plano lens, in that orderfrom the front-object side. Besides, an aperture stop S is provided onthe front-object side of the lens L₃₁ and integratedly with the lensL₃₁. Also, the lens L₃₃ and the lens L₃₄ that is a biconcave lens arejointed together to constitute a cemented lens.

Also, the lens barrel which holds the optical system, thelight-shielding member, and the imaging element is composed of a firstlens barrel T₁ and a second lens barrel T₂.

The first lens barrel T₁ holds the first lens group G₁ and the secondlens group G₂. More specifically, the first lens barrel T₁ holds thefirst lens group G₁ on the inside of first lens barrel T₁. Also, thefirst fitting portion L₂₁d of the lens L₂₁ that is the optical elementis fitted to the first lens barrel T₁ on the image side of the firstlens barrel T₁, so that the first lens barrel T₁ holds the lens L₂₁.That is to say, a part of the internal peripheral surface of the firstlens barrel T₁ comes into contact with the peripheral surface of thelens L₁₁. Also, a part of the internal peripheral surface of the firstlens barrel T₁ and the image-side plane of the first lens barrel T₁ comeinto contact with the first fitting portion L₂₁d.

On the other hand, the second lens barrel T₂ holds the second lens groupG₂, the light-shielding member F, the third lens group G₃, and theimaging element. More specifically, the second fitting portion L₂₁e ofthe lens L₂₁ that is the optical member is fitted to the frontobject-side edge plane of the second lens barrel T₂, so that the secondlens barrel T₂ holds the lens L₂₁. Also, the second lens barrel T₂ holdsthe lens L₂₂ on the inside of the second lens barrel T₂. That is to say,a part of the internal peripheral surface of the second lens barrel T₂comes into contact with the peripheral surface of the lens L₂₂. Also,the front object-side plane of the second lens barrel T₂ comes intocontact with the second fitting portion L₂₁e.

As described above, a fitting portion which is fitted to the lens barrelis provided for the lens L₂₁ that is the optical element in the opticalunit of the present embodiment, so that the lens L₂₁ that is the opticalelement can be fitted directly to the lens barrel. As a result, there isno necessity for providing the optical unit of the present embodimentwith a holding portion which is larger than the diameters of lenses ofthe optical system as in prior optical units, and it is possible todownsize the whole of the apparatus, as compared with prior opticalunits.

Besides, the first and second lens barrels T₁ and T₂ are formed in sucha way that the external peripheral surfaces of the first and second lensbarrels T₁ and T₂ approximately correspond with the external peripheralsurface of the third surface of the lens L₂₁. And, these surfaces formthe external peripheral surface of the optical unit.

Also, the annular light-shielding member F which is inserted into theoptical system is arranged nearer to the object side than the aperturestop S. Also, the light-shielding member F is formed in such a way thatthe internal diameter of the light-shielding member F is larger thanthat of the aperture stop S.

Besides, the light-shielding member F is made of light-proof material.Also, the light-shielding member F is formed in such a way that theexternal diameter of the light-shielding member F approximatelycorresponds with the internal diameter of the second lens barrel T₂, thelight-shielding member F is fitted to the second lens barrel T₂ in sucha way that a gap does not occur, and the light-shielding member F isfixed to the second lens barrel T₂ with a light-proof adhesive.

As described above, the optical unit of the present embodiment comprisesthe optical system by which a front object and an approximately lateralobject can be simultaneously observed with the lens L₂₁ that is theoptical element, so that it is possible to restrain the occurrence ofstray light inside the optical system, as compared with prior opticalunits. That is to say, the light-shielding member F which is made oflight-proof material is arranged nearer to the image side than thesecond lens group G₂ which includes the lens L₂₁ that is the opticalelement, so that it is hard for stray light occurring nearer to theobject side than light-shielding member F to reach the image plane. As aresult, it is possible to effectively reduce flare and ghost.

Next, a variation of the optical unit according to the embodiment 2 isexplained using FIG. 8. In this optical unit which is different from theoptical unit of the embodiment 2, the light-shielding member F isarranged between: the lens L₃₁ nearest to the object side in the thirdlens group G₃; and the lens L₃₂ on the image side of the lens L₃₁. Sucha constitution also makes it possible for the optical unit of thisvariation to have the same effects as those in the optical unit of theembodiment 2.

Next, another variation of the optical unit according to the embodiment2 is explained using FIG. 9. In this optical unit which is differentfrom the optical unit of the embodiment 2, the shape on the inside ofthe second lens barrel T₂′ that constitutes the lens barrel is formed inthe shape of a step so that the internal diameter of the portion of thesecond lens barrel T₂′ which holds the light-shielding member F islarger than the image-side internal diameter of the second lens barrelT₂′.

As described above, the portion which holds the light-shielding member Finside the lens barrel is formed in the shape of a step, so that, inthis optical unit, stray light which occurs nearer to the object sidethan the light-shielding member F does not enter the third lens group G₃and imaging element which are nearer to the image side than thelight-shielding member F, through a gap between the light-shieldingmember F and the lens barrel which occurs due to an assembly error orthe like. As a result, it is possible to reduce the occurrence of flareand ghost more effectively. In addition, it becomes easy to assemble theoptical unit.

Next, another variation of the optical unit according to the embodiment2 is explained using FIG. 10. In this optical unit which is differentfrom the optical unit of the embodiment 2, the second lens barrel T₂″which constitutes the lens barrel is composed of an external barrel T₂″,and an internal barrel T₂″₂ that is held on the inside of the externalbarrel T₂″₁. Also, the light-shielding member F is integratedly formedin the object-side edge plane of the internal barrel T₂″₂. Besides, bothof the internal barrel T₂″₂ and the light-shielding member F that isformed integratedly with the internal barrel T₂″₂ are made oflight-proof material.

As described above, the light-shielding member F is formed integratedlywith the internal barrel T₂″₂ which holds the third lens group G₃, sothat, in this optical unit, stray light which occurs nearer to theobject side than the light-shielding member F does not enter the thirdlens group G₃ and imaging element which are nearer to the image sidethan the light-shielding member F, through a gap between thelight-shielding member F and the internal barrel T₂″₂ which occurs dueto an assembly error or the like. As a result, it is possible to reducethe occurrence of flare and ghost more effectively. Also, it alsobecomes easy to assemble the optical unit. In addition, the third lensgroup G₃ can be assembled separately from the other lens groups, so thatit also becomes easy to assemble the optical unit and to adjusteccentricity for the optical system.

Besides, in this optical unit, although the second lens barrel T₂″ iscomposed of the external barrel T₂″₁ and the internal barrel T₂″₂ thatis held on the inside of the external barrel T₂″₁, the second lensbarrel T₂″ of this optical unit is not limited to such a constitution,and the second lens barrel T₂″ may be composed of three or more members.

Next, another variation of the optical unit according to the embodiment2 is explained using FIG. 11. In this optical unit which is differentfrom the optical unit of the embodiment 2, the aperture stop S isintegratedly formed between a position at which the second lens barrelT₂′″ constituting the lens barrel holds the light-shielding member F anda position at which the second lens barrel T₂′″ holds the third lensgroup G₃. Also, the second lens barrel T₂′″ is formed in such a way thatthe internal diameter of the second lens barrel T₂′″ which is nearer tothe object side than the integratedly formed aperture stop S is largerthan the image-side internal diameter of the second lens barrel T₂′″.

As described above, the aperture stop S is formed integratedly with thesecond lens barrel T₂′″, so that, in this optical unit, stray lightwhich occurs nearer to the object side than the light-shielding member Fdoes not enter the third lens group G₃ and imaging element which arenearer to the image side than the light-shielding member F, through agap between the light-shielding member F and the second lens barrel T₂′″which occurs due to an assembly error or the like. As a result, it ispossible to reduce the occurrence of flare and ghost more effectively.Also, it also becomes easy to assemble the optical unit. In addition,the third lens group G₃ can be assembled separately from the other lensgroups, so that it also becomes easy to assemble the optical unit and toadjust eccentricity for the optical system.

Besides, the present invention is not limited to these examples, and thepresent invention also includes various combinations of theabove-described embodiments and variations.

Also, in the above-described embodiments, the optical systems by which afront object and an approximately lateral object can be simultaneouslyobserved are composed of three lens groups. However, optical systemswhich are provided for optical units of the present invention are notlimited to these examples, and the optical systems may be composed oftwo lens groups or composed of four or more lens groups.

Also, lenses which constitute lens groups of the optical systems thatare provided for optical units of the present invention are not limitedto the shapes and the numbers of the lenses which are described in theabove-described embodiments, and optical units of the present inventionalso includes various optical systems including the optical element.

Also, in the above-described embodiments and variations, thelight-shielding members are arranged nearest to the object side in thethird lens group, or on the image side of the lens nearest to the objectside in the third lens group. However, a position of light-shieldingmember arranged is not limited to the above-described positions.Besides, in order to improve an effect of restraining flare and ghost,it is preferred that the light-shielding members are arranged at aposition which is nearer to the object side than the lens nearest to theimage side in the third lens group.

Optical units according to the present invention as described above canbe used for imaging apparatuses, in particular, such as imagingapparatus for endoscope, in which imaging is performed by forming on animaging element like CCD the image of an object that is formed by anoptical system included by each of the optical units. One specificexample of the imaging apparatuses is illustrated below.

An endoscope apparatus 1 includes: an elongated insertion unit 11 whichis inserted into a body cavity; a light source device (light sourceunit) which is not shown in the drawings; a camera control unit (CCU) 12which detects light forming an image in the top end of the insertionunit 11 to image the light and performs image processing; and a monitor13 which displays an image that is given image processing by the CCU 12,as shown in FIG. 12.

The insertion unit 11 is provided with a light guide (illuminationoptical system) for guiding light from the light source device toirradiate the light to observation objects such as inside surface of abody cavity, which is not shown in the drawings. In addition, an opticalunit 11 a according to the present invention and an imaging element 11 bwhich captures an image of an observation object that is formed by anoptical system in the optical unit 11 a are provided in the top endportion of the insertion unit 11.

The light source device includes: a light source which emits light in arelatively wide wavelength range, for example, like xenon lamp, halogenlamp, white LED, or near-infrared LED; and a filter turret whichselectively extracts light of plural kinds of wavelengths from lightemitting from the light source.

The CCU 12 includes a control circuit 12 a which maps timing of a changeof respective filter components of the filter turret onto an imageacquired by the imaging element 11 b; and an image processing unit 12 bwhich processes the acquired image.

Besides, the imaging element 11 b may be provided not in the top endportion of the insertion unit 11 but inside the CCU 12. In this case, animage guide which guides light from the optical system in the range fromthe image side of the optical unit to the base end-side of the insertionunit 11 should be provided.

INDUSTRIAL APPLICABILITY

An optical element according to the present invention and an opticalunit using the same can restrain the deterioration of the capability offorming an image, so that the optical element and the optical unit canbe preferably used for endoscopes or the like and are extremely usefulfor practical use.

1. An optical element, wherein the optical element is provided with afirst surface which is formed on the front-object side and through whichlight from the front-object side enters, a second surface which isformed on the image side, and a third surface which is formed betweenthe first and second surfaces and through which light from theapproximately-lateral-object side enters, the first surface is providedwith a first transmission surface which is formed with the center of thefirst transmission surface being on the optical axis and a firstreflection surface which faces toward the image side and is formedaround the first transmission surface and in the shape of a ring, andthe second surface is provided with a second transmission surface whichis formed with the center of the second transmission surface being onthe optical axis and a second reflection surface which faces toward thefront-object side and is formed around the second transmission surfaceand in the shape of a ring.
 2. An optical element according to claim 1,wherein after light from the front-object side is incident on the firsttransmission surface, the light emerges from the second transmissionsurface to the image side, and after light from theapproximately-lateral-object side is incident on the third surface, thelight is reflected by the second reflection surface and the firstreflection surface in that order and emerges from the secondtransmission surface to the image side.
 3. An optical unit, wherein theoptical unit is provided with an optical system by which a front objectand an approximately lateral object are observed, and a lens barrelwhich holds the optical system, the optical system includes the opticalelement according to claim 1, the lens barrel is provided with a firstlens barrel and a second lens barrel, the first lens barrel is fitted tothe first surface of the optical element, and the second lens barrel isfitted to the second surface of the optical element.
 4. An optical unitaccording to claim 3, wherein the external peripheral surface of theoptical unit is composed of the external peripheral surface of the lensbarrel and the third surface.
 5. An optical unit according to claim 3,wherein the optical system comprises a first optical system and a secondoptical system, the first optical system is provided with a first lensgroup having negative refractive power, a second lens group includingthe optical element, and a third lens group in order from thefront-object side, and the second optical system is provided with thesecond lens group and the third lens group in order from theapproximately-lateral-object side.
 6. An optical unit according to claim3, wherein a first fitting portion is formed around the first reflectionsurface of the optical element, a second fitting portion is formedaround the second reflection surface of the optical element, the firstlens barrel is fitted to the first fitting portion, and the second lensbarrel is fitted to the second fitting portion.
 7. An optical unitaccording to claim 6, wherein at least one of the first and secondfitting portions is formed in the shape of a step.
 8. An optical unitaccording to claim 7, wherein the first and second fitting portions areformed in such a way that the first and second fitting portions arerotationally symmetric relative to the optical axis.
 9. An optical unitaccording to claim 3, wherein the first lens barrel includes a firstholding portion, the second lens barrel includes a second holdingportion, the first holding portion holds the optical element through thefirst surface of the optical element and the second holding portionholds the optical element through the second surface of the opticalelement, and at least one of the first and second holding portions isformed in the shape of a step.
 10. An optical unit according to claim 9,wherein the first and second holding portions are formed in such a waythat the first and second holding portions are rotationally symmetricrelative to the optical axis.
 11. An optical unit, wherein the opticalunit comprises an optical system by which a front object and anapproximately lateral object are observed, a lens barrel which holds theoptical system, and an imaging element, the optical system includes alens group which includes the optical element according to claim 1 and alens group which is arranged nearer to the image side than the lensgroup including the optical element and includes an aperture stop, andthe optical unit is provided with an annular light-shielding memberwhich is placed in the vicinity of the aperture stop, is made oflight-shielding material, and has an internal diameter that is largerthan the internal diameter of the aperture stop and smaller than theinternal diameter of the lens barrel.
 12. An optical unit according toclaim 11, wherein the aperture stop is formed integratedly with thelight-shielding member.
 13. An optical unit according to claim 11,wherein the imaging element is held by the lens barrel.
 14. An opticalunit according to claim 11, wherein the lens barrel consists of aninternal barrel which holds a lens group that is arranged nearer to theimage side than the optical element, and an external barrel which holdsthe internal barrel, and the light-shielding member is formedintegratedly with the internal barrel.
 15. An optical unit according toclaim 11, wherein the external diameter of the light-shielding memberapproximately corresponds with the internal diameter of the lens barrel.